Although ZnO and its related heterostructures are really attractive for their potential application in optoelectronics, their developments have been limited by the p-type doping issue. Here, we will show why ZnO properties are also very attractive for unipolar structures, only dealing with electrons, and how the material quality has been improved to reach these devices requirements.
First, the benefit of homoepitaxy through material quality improvement is presented. We will show that molecular beam epitaxy allows getting defect density, surface roughness, and residual doping, comparable to the state-of-the-art of GaAs. Moreover, (Zn,Mg)O alloy could be used to fabricate heterostructures with very good optical and transport properties.
In the second part, we will give a brief overview of the main transport results, especially bidimensional electron gas, reported in the literature. Few examples of possible applications will also be addressed. Then, we will focus on the potentialities of nonpolar ZnO heterostructures for unipolar devices based on intersubband transitions. Once the advantages of using ZnO for TeraHertz quantum cascade laser discussed, we will show that the structural properties of the ZnO/(Zn,Mg)O heterostructures fulfill the requirements of these devices operation. Moreover, we will finish with absorption measurements clearly showing intersubband transitions in agreement with the light polarization selection rule. The strong influence of physical parameters, like doping level, on the energy of these kind of transitions will also be discussed.
This work was funded by EU commission under the H2020 FET-OPEN program; project “ZOTERAC” FET-OPEN 6655107.